High-voltage electrolytic capacitor

ABSTRACT

An electrolytic capacitor suitable for operating at high voltages is produced by combining a glycol-borate electrolyte together with N-methyl pyrrolidone and a moderately hydrogenbonding solvent. The electrolytic capacitor has a wide range of temperature conditions, operates at high voltages and retains stable electrical characteristics at these levels.

United States Patent Dunkl 1 Feb. 29, 1972 s41 HIGH-VOLTAGE ELECTROLYTIC CAPACITOR [72] inventor: Franz S. Dunk], Williamstown, Mass.

[73] Assignee: SpragueElectric Company, North Adams,

Mass.

[221 Filed: Oct. 23, 1970 211 App1.No.: 83,568

[52] US. Cl ..3l7/230, 252/622 [51] Int. Cl. ..1-l0lg 9/00 [58] Field of Search ..317/230, 231, 233

[56] References Cited 9 UNITED STATES PATENTS 2,965,690 12/1960 Petersen et a1. ..317/230 3,138,746 6/1964 Burger et al. ..317/230 3,302,071 l/l967 Stahr 317/230 3,351,823 11/1967 Jenny.... 317/230 3,504,237 3/1970 Stahr 317/230 Jenny et al...

Primary Examiner-James D. Kallam Att0rneyConnol1y and Hutz and Vincent H. Sweeney [57] ABSTRACT An electrolytic capacitor suitable for operating at high voltages is produced by combining a glycol-borate electrolyte together with N-methyl pyrrolidone and a moderately hydrogen-bonding solvent. The electrolytic capacitor has a wide range of temperature conditions, operates at high voltages and retains stable electrical characteristics at these levels.

4 Claims, 1 Drawing Figure HIGH-VOLTAGE ELECTROLYTIC CAPACITOR BACKGROUND OF THE INVENTION This invention relates to high voltage electrolytic capacitors, and more particularly to a glycol-borate electrolyte con 'taining-hydrogen-bonding solvents.

1 Glycol-borate electrolyteshave been used frequently in the past because of their many advantages. These advantages are wellknown in the art, but they have many disadvantages such as the fact that their voltage capability is limited to approxiaqueous electrolytic capacitors have been made with some success; see US. Pat. Nos. 3,351,823 and 3,302,071. The electrolytes contained therein have combined N-methyl pyrrolidone and butyrolactone. This combination, to some extent overcomes the disadvantages outlined above. However, these same electrolytes cannot operate about 450 volts because of A scintillation and eventual breakdown.

Accordingly, it is an object of the present invention to increasethe voltage capability of the capacitor, to extend the temperature range of glycol-borate electrolytes, and to maintain theresistivity at levels now found in the glycol-borate electrolytes.

SUMMARY OF THE INVENTION The-present invention is concerned with a standard glycolborate electrolyte and the addition thereto of N-methyl pyrrolidone and a moderately hydrogen-bonding solvent. It has been found that the addition of N-methyl pyrrolidone to the standard mixture electrolyte reduces the water activity of same without destruction of the borate complex which is necessary for conductivity and forming ability.The further addition of amoderately hydrogen-bonding solvent, such as 2- methoxy ethanol, lowers the freezing point of the electrolyte,

decreases the viscosity and consequently lowers the resistivity while retaining high voltage capabilities. This combination results in an electrolytic capacitor that has a capacity of 380 -;.if., a'dissipation factor of 9 percent, and a leakage current of Glycol -45% Ammonium pentaborate 15-25% Boric acid 5-l3% Ammonium dihydrogen phosphate 0-0.B% N-mcthyl pyrrolidone 15-20% Z-mclhoxy ethanol 15-20% BRIEF DESCRIPTION OF THE DRAWING The-FIGURE shows the rolled plates of an electrolytic capacitor.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the FIGURE shown there is a capacitor assembly. Ill-having a pair of electrode foils 11 and 12 separated by spacer paper Band 14. Electrode tabs 15 and 16 are in electrical communication with foils lland 12.

The rolled 'foil-spacer-foil system is impregnated with the electrolyte. 'T he convolutely wound capacitor section 10 is then inserted into a suitable containerv and sealed.

A life test was carried out on an electrolytic capacitor of the present invention whose electrolyte composition was as given in the following formulation wherein all parts are percent by weight:

Glycol Ammonium pentaboratc- 19.4%

' Boric acid 9.4%

Ammonium dihydrogen phosphate 7 0.1%

N-methyl pyrrolidone 17.3%

.Z-methoxy ethanol The life test was carried out on this capacitor at 500 v. DC and C. and gave the following results:

As was pointed out previously, capacitors impregnated with this electrolyte had a capacity of 380 microfarads, a dissipation factor of 9 percent and a leakage current of l milliamp at 500 volts and 105 C.

The electrolyte of the present invention has better properties than the previously employed solutions using nonaqueous solvents, as well as the standard glycol-borate electrolytes. For example, these electrolytes have the desirable-property of broad temperature range of operation as well as the capability of operating at high voltage and high temperature atmospheres, while maintaining approximately the same resistivity levels. It should be emphasized at this point that the present invention is capable of operating at high voltages, while prior art attempts in this area have only been capable of operating efficiently below 450 volts.

While 2-methoxy ethanol is employed in the preferred embodiment of this invention as the moderately hydrogen-bonding solvent, it should be noted that other hydrogen-bonding solvents may be used with somewhat similar results. Hydrogen-bonding solvents will modify the properties of the resulting electrolyte; for example, to decrease the viscosity of the electrolyte or lower the freezing point thereof, or increase the boiling point therein. Obviously, by using-these combinations one may alter or vary the temperature range, voltage capability and resistivity of the units.

Other moderately hydrogen-bonding solvents that can be used herein, with similar but less desirable results are: diethylene glycol monomethyl ether, 2-.ethoxy ethanol, 3-heptanone, 2,6-dimethyl 4-heptanone, and 2-methoxyethyl acetate.

Also, in the preferred embodiment herein the 2-methoxy ethanol is present in the amount of 17.3 percent. This amount was chosen because although higher amounts contributed to higher voltage capabilities at the higher temperatures, it also caused a proportionately higher leakagecurrent at room temperature. And amounts smaller than that preferred herein, resulted in proportionately lower voltages and higher resistivities. However, amounts around the 15-20 percent range can be used.

A 1:1 ratio is used herein for the N-methyl pyrrolidone and Z-methoxyethanol; it was found that when other moderately hydrogen-bonding solvents were used, the ratio remained about the same for best results. The deciding factors considered therein are to keep the resulting boiling point high, the freezing point low, and .the resistivity relatively low.

Butyrolactone has been employed in prior art electrolytes with N-methyl pyrrolidone, but it appears that .butyrolactone is not as strong a hydrogen-bonding solvent as is needed to achieve :the higher voltages. Also, although ,I do not wish to be bound by ethe following, it appearsthat the vapor pressure of butyrolactone, as compared with the hydrogen-bonding solvents disclosed herein, is relatively high and probably causes loss of electrolyte and a less efficient electrolyte.

When N-methyl pyrrolidone is used alone in a glycolborate electrolyte, the resistivity of the solution is too high to be efficient. This is one of the reasons for using a moderately hydrogen-bonding solvent.

The ionogen component used in the electrolyte mixture of the invention is generally present in relatively small amounts and ordinarily an amount thereof is employed which is sufficient to provide adequate resistivity of the electrolyte for the voltage applied to the capacitor in operation.

While the present invention has been described with reference to particular embodiments thereof it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the scope of the invention. Therefore the appended claims are intended to cover all such equivalent variations as common within the true spirit and scope of the invention.

What is claimed is:

l. A high voltage electrolytic capacitor comprising a formed anode electrode of a valve metal, a cathodecontact electrode, and a liquid electrolyte in contact with said electrodes, said electrolyte comprising a glycol, a borate, N-methyl pyrrolidone, and at least one moderately hydrogen-bonding solvent selected from the group consisting of 2-methoxy ethanol,

Glycol 30-45% Ammonium pentaborate l5-2SI: Boric acid 5-] 3% Ammonium dihydrogen phosphate 04.8% N-methyl pyrrolidone lS-ZOk Z-methoxy ethanol 15-20%.

4. The electrolytic capacitor of claim 2 wherein said electrolyte comprises a mixture in percent by weight of:

Glycol v 36.5% Ammonium pentaborate l9.4% Boric acid 94% Ammonium dihydrogcn phosphate 0.l% N-methyl pyrrolidonc 17.3% Z-methoxy ethanol l7.3%. 

2. The capacitor of claim 1 wherein said electrolyte includes ammonium dihydrogen phosphate and said borate is ammonium pentaborate.
 3. The electrolytic capacitor of claim 2 wherein said electrolyte comprises a mixture in percent by weight of: Glycol 30-45% Ammonium pentaborate 15-25% Boric acid 5-13% Ammonium dihydrogen phosphate 0-0.8% N-methyl pyrrolidone 15-20% 2-methoxy ethanol 15-20%.
 4. The electrolytic capacitor of claim 2 wherein said electrolyte comprises a mixture in percent by weight of: Glycol 36.5% Ammonium pentaborate 19.4% Boric acid 9.4% Ammonium dihydrogen phosphate 0.1% N-methyl pyrrolidone 17.3% 2-methoxy ethanol 17.3%. 